1. Field of the Invention
The present invention relates to a disk drive for reproducing information from a disk-shaped recording medium, such as a magnetooptical disk, an optical disk and the like, and/or recording information thereon. More specifically, the present invention relates to a signal processing section for processing a signal read from the recording medium.
2. Description of the Related Art
On a recording surface of a disk-shaped recording medium (hereinafter, simply referred to as a disk), for example, a magnetooptical disk having a diameter of 130 mm (hereinafter, simply referred to as a 130 mm magnetooptical disk), a region is provided for storing various kinds of information, including: servo methods; a rotation mode; the type of disk, i.e., whether the disk is of a read-only type, a write-once type, or a rewritable type; a modulation method; an error correction code; a number of user bytes in one sector, and the like. This region is called a "PEP region".
The data format of the PEP region is determined in accordance with the standard. As shown in FIG. 5, three gap parts where no signal is recorded and three sector parts where various kinds of information are recorded are alternately arranged per one rotation of the disk. The PEP region is composed of 561 to 567 bit cells per one rotation of the disk. A "bit cell" herein refers to a unit length of the region in which one unit of information representing either "0" or "1" is recorded.
As shown in FIG. 6, a sector part includes: a preamble section composed of 16 bit cells and used as a sector start buffer; a sync section composed of 1 bit cell and used for the detection of synchronization; a sector number section composed of 8 bit cells and used for the identification of a sector number; a data section composed of 144 bit cells and used for writing data therein; and a CRC section composed of 8 bit cells and used for the error detection.
The units of information recorded in the PEP region are discriminated one by one, as shown in FIG. 7, thereby various kinds of information are reproduced. More specifically, when the state where a recording mark is present (hereinafter, referred to as a "mark-present state") is turned into a state where a recording mark is absent (hereinafter, referred to as a "mark-absent state") in one bit cell, or when the level of a reproduced signal falls in a period corresponding to one bit cell, the unit of information carried by the reproduced signal is discriminated as "0". On the other hand, when the mark-absent state is turned into the mark-present state in one bit cell, or when the level of a reproduced signal rises, the unit of information carried by the reproduced signal is discriminated as "1".
For example, in the preamble section following the gap part where no signal is recorded, 16 units of information representing "0" are successively recorded, as shown in FIG. 8. In the sync section following the preamble section, one unit of information representing "1" is recorded.
In the PEP region, it depends upon the presence of the recording mark in one bit cell whether the unit of information corresponding to the one bit cell represents "0" or "1". The units of information are obtained by dividing the signal reproduced from the disk into a plurality of portions each of which corresponds to one bit cell and then interpreting each of the divided portions. Therefore, unless the signal reproduced from the PEP region (hereinafter, simply referred to as a "PEP signal") is correctly divided, the information recorded in the PEP region cannot be correctly reproduced.
In a conventional disk drive, in order to correctly reproduce the information recorded in the PEP region, the PEP signal starts to be read after the rotation speed of the disk increases and is stabilized at a constant value. While the rotation speed of the disk is constant, the time period required for reading the signal corresponding to one bit cell is also constant. Therefore, by starting to divide the PEP signal into a plurality of portions immediately after the unit of information representing "1" in the sync section is detected, the PEP signal can be correctly divided. As a result, various kinds of information recorded in the PEP region can be correctly reproduced.
However, in the case where the reproduction of the PEP signal is not started until the rotation speed of the disk becomes constant, as in a conventional disk drive, the period after the disk starts to rotate until the rotation speed of the disk becomes constant is a waste for reproducing the information recorded in the PEP region. Accordingly, such a period prevents the reproduction speed or the recording speed for the disk from being increased.
Instead, if an attempt is made to reproduce the information recorded in the PEP region during a period after the disk starts to rotate until the rotation speed of the disk becomes constant, then the period corresponding to one bit cell changes in accordance with the increase in the rotation speed of the disk. Therefore, the PEP signal cannot be correctly divided. As a result, it is also impossible to correctly reproduce various kinds of information recorded in the PEP region.